Photovoltaic solar cells have to extract high current densities, in the range of 10 to 50 mA/cm2 of current per unit area perpendicular to the solar illumination. Such a device is illustrated in FIG. 1 in a cross-sectional view. The generated photocurrent is usually collected and transported laterally through a conductive front layer 1, such as a highly doped region of silicon, a thin transparent metallic layer, or a transparent conductive oxide such as SnO2, ITO, ZnO, InO:H (Indium oxide) or a combination of those. Eventually the current is collected by more conductive contact lines called fingers or grid contacts 2. FIG. 5 shows a typical metal grid pattern placed on a front side of a photovoltaic solar cell. Busbars 3 are usually contacted (soldered, welded or contacted with electrical glue) with a highly conductive metal string to extract the current that is generated homogeneously over the optically active area (i.e. not covered by the contact grid) and collected by the thin fingers or grid contact 2. These grid contacts are often deposited using the screen-printing of conductive paste, such as silver containing paste.
If the screen-printed contacts are realized only by screen-printing or related techniques (e.g. stencil printing), using a single or a double print, they tend to be wide (70-120 μm), thereby shadowing the solar cells and creating optical losses, as typically 4-11% of the surface of the solar cell is then covered by a reflective material (meaning area where no photon can be collected). Also the bulk electrical resistivity of the screen-printed paste (typically Ag flakes with inorganic and organic components) is significantly higher than the one of the corresponding bulk material. In the case of Ag, the resistivity for a paste which is cured at high temperature)(700-900° is a factor 2-3 higher than the one of bulk Ag, and 7-10 higher in the case of pastes cured at low temperature (150-300°). This low conductivity can lead to strong electrical losses in the contact fingers of the photovoltaic solar cell. In order to compensate for the low conductivity, thick fingers have to be printed (typically several tens of micrometers). The disadvantage is that it can still be insufficient and still produces electrical losses, and, in addition, there is the need to use large quantity of screen-printed paste, which can be an expensive component.
As shown by FIG. 2, one possibility to limit both the optical losses, the electrical losses and to save material, would be to deposit on the conductive front layer 1, by screen-printing or another technique, a thinner seed-line 4, and subsequently to reinforce the electrical contact by a plating process (electroless plating, or electroplating), typically using Ag, Cu, Ni and other layers for plating the contacts. The advantage of plated material 5 is a higher conductivity, closer to the bulk conductivity of the used materials.
The problem is that, in the proposed configuration, both the seed-line or grid contact 4, and the underlying layer 1 are conductive. Hence the materials 5 deposited by plating will tend to also deposit on both the seed-line 4 and the underlying layer 1, or at least partially on the underlying layer 1. This will then lead to undesired metal coverage of the cell front surface, leading to optical losses. It is possible to work by varying the composition of the plating solutions to achieve a “selectivity” between the finger contacts 4 and the underlying layer 1, but it will also be difficult to warrant a robust process in any production, because in principle all conductive layers will be able to give or accept the electrons needed in the plating process (and furthermore with electroplating).
One object of the present invention is to provide a device, such as a solar cell, which allows to avoid the disadvantages of the prior art. More specially, one object of the invention is to provide a device comprising a conductive surface and electrical contacts, said electrical contacts having a reduced size and an increased conductivity.